The present invention relates to a high gain crossed field amplifier tube.
Crossed field amplifier tubes are generally used in the power stage of radar transmitters. The invention also relates to radio transmission systems equipped with such a tube.
These tubes are essentially constituted by two cylindrical, concentric electrodes placed under vacuum between which a potential difference is produced, which creates a d.c. field E.sub.o. A magnetic field B is applied parallel to the tube axis and therefore perpendicular to the electric field.
The internal electrode is a cathode forming an electron current source. The external electrode is a delay line, whose function is to propagate the high frequency wave with a phase velocity V.sub..phi. of the order of a fraction of the speed of light.
Under the interconnected actions of the electric field and the magnetic field, the electrons from the cathode follow cycloidal trajectories with an average azimuth speed V.sub.e.
It has been shown that the amplification of the high frequency power occurs when V.sub.e =V.sub..phi.. A distinction is made between crossed field tubes with a forward or backward wave, as a function of whether the high frequency energy flows in the direction of the electron beam or in the reverse direction. The gain of crossed field amplifiers is given by the expression g=101og(Ps/Pe).
Increasing the gain means decreasing the input lower Pe or increasing the output power Ps. In connection with the first solution there is a minimum value of Pe below which the tube does not operate because the power is insufficient for creating the first space charge branch. This value is dependent on the geometrical characteristics of the delay line, electrical and magnetic characteristics and the secondary emission coefficient of the cathode.
If Pe is equal to the minimum Pe the gain can reach 18 dB, but in this case the signal to noise ratio is too low (&lt;20 dB). To bring this ratio to an acceptable value (approximately 40 dB) it is necessary to slightly increase the input power. In this case the value of the gain hardly exceeds 13 dB.
Another way to increase the gain is to increase the output power Ps, which is given by the relation Ps=.eta..I.Uc, .eta. being the overall efficiency of the tube which is approximately 50%.
The output power Ps is consequently essentially proportional to the electron current I, the operating voltage Uc varying only very slightly with the current. The total current I is proportional to the number N of space charge branches, each branch transmitting a current substantially equal to I/N.
If an attempt is made to increase the gain of a crossed field amplifier by doubling, for example, the length of the line the improvement is at the most 3 dB. Thus, this operation is ineffective and can lead to a lack of stability, because the interfering modes are sensitive to the line length.
On attempting to obtain the same result by increasing the operating current so as to double the total current, there is a multiplication by two of the current transmitted in each space charge branch, including the first and it is necessary to increase the input power in the same proportions for stabilizing said branch. Thus, it is not advantageous to increase the gain in this way.